The invention relates to a measuring arrangement for measuring the density of a product by means of microwaves.
Certain known devices comprise a first microwave resonator, from which, in operation, microwaves enter a product area. When measuring density using such a measuring arrangement, the measuring variables used, in particular the resonant frequency and the width of the resonance curve, are dependent on different environmental influences and disturbance variables in the first resonator. In order to compensate for the influences, such as the temperature of the resonator, on the systematic measuring error, it is known to provide a temperature sensor at a specific point of the first resonator and to carry out a computational correction by means of the measured temperature. Since the temperature in the first resonator is determined only at one point, the accuracy of the compensation in the first resonator is limited, especially in the case of a spatially non-homogeneous and/or time-variant temperature distribution.
Generally speaking, to compensate, for example, for the effects of temperature, it is known to use different materials, in the present case for the first resonator, having temperature coefficients of opposite signs. Apart from increased manufacturing costs, however, the transition points between the different materials in particular can have an adverse effect in microwave technology. The use of special alloys and composite materials having thermal expansion coefficients close to or equal to zero is also known. These too involve increased manufacturing costs.
It is an aim of the invention to provide an apparatus and method of the kind mentioned above in which the measuring error caused by different environmental influences and by internal and external disturbance variables in the first resonator can be reduced.